They did away with the annual 'Ready Mobilization Pool' list they had for a few years last year so folks are just getting notified of their mobs the old way, they get a few months notice out of the blue to deploy. But that isn't such a drastic change, they are usually giving plenty of notice and they aren't mobilizing anywhere near the numbers they did just a few years ago. Plenty of the folks currently getting mob'd are no kidding involuntarily mob'd (contrary to what I had heard a year or so ago about having enough volunteers but that was just RUMINT, not official) but that shouldn't be a surprise to anyone who joins the reserves nowadays or who has been in and awake the last few years.
Perhaps part of the problem is that what you describe is happening, but that's not what's supposed to be happening. The original plan (at least within CNAFR) was at the beginning of the "RMP" year, COs would notify the SELRES that were eligible to be MOB'ed so they were prepared, even if they were never actually called. It sounds like this morphed into a system where sometimes the CO would play "I have a secret" and then spring it on a SELRES. There's various reasons that might occur.
I can say that this tactic is definitely not everywhere.
But I really don't see how the recent change would be significant enough to cause a 'mass exodus' unless they are doing something different with the hardware units who I know were protected to a degree under the old systems. It is actually a return to the 'old' way of doing things before the Ready Mob Pool but better run and with a lot less people getting mob'd. Personally I haven't seen an exodus at all and though my view is limited to a few units they are pretty aviation heavy with quite a few folks.
It seems like it's hit the VR guys a good bit, talking with some buds who are VR. And if people know their in the local pool ahead of time, then they may just drop their letter before the notification can occur (if it actually does), which then makes the next junior guy eligible, so he bails, etc. The fact that the COMAIR landscape is much healthier now probably has a lot to do with it.